As is well known, the foundry art is that art dealing with the formation of metal articles by casting processes wherein molten metal is poured into a mold, allowed to cool, and solidify. By far the largest quantity of castings are made by processes in which the mold is formed from sand, i.e., by sand casting processes. There are several different sand casting processes, but the one employed most often is that employing green molding sand.
Green molding sand has been defined as a "plastic mixture of sand grains, clay, water and other materials which can be used for molding and casting processes. The sand is called `green` because of the moisture present and is thus distinguished from dry sand." (Heine et al., "Principles of Metal Casting," McGraw-Hill Book Co., Inc., New York (1955), p. 22). Green sand has also been defined as "a naturally bonded sand or a compounded molding-sand mixture which has been tempered with water for use while still in the damp or wet condition." ("Molding Methods and Materials," 1st. Ed., The American Foundrymen's Society, Des Plaines (1962)). Such a sand contains water or moisture both in the mold-forming stage and in the metal casting phase. As employed herein, the term "foundry green molding sand" has reference to green molding sands of the type known to and employed by those of ordinary skill in the foundry art comprising molding sand and clay and tempered with water.
As is evident from the foregoing, the essential components of a foundry green molding sand are molding sand, clay and water. The molding sand, which usually is a silica sand (e.g., quartz), but which may be a zircon, olivine or other refractory particulate material having mesh sizes commonly in the range of from about 6 to about 270 mesh, serves largely as a filler and provides the body of the mold. The clay, which is a finely divided (normally less than about 2 microns) material such as montmorillonite (bentonite), illite, kaolinite, fire clay and the like, when plasticized with water, serves as a binder for the sand grains, and imparts the physical strength necessary to enable use of the green molding sand as a mold material. Ordinarily, green molding sands contain from about 5 to about 20 weight percent clay, based upon sand, and sufficient water, normally not greater than about 8 weight percent, based upon sand, to achieve the desired plasticity and other physical properties. The amount of temper water normally is greater when naturally-bonded sands are employed than when synthetic sands are employed.
There are a number of properties which are desired in foundry green molding sands. Among the most important are:
1. Good flowability or compactibility to allow the sand to move against the pattern under compacting forces; PA1 2. Good physical strength after compaction to permit the mold to retain its shape after removal of the pattern and during casting; PA1 3. Dimensional stability during the casting process; PA1 4. Good internal cohesion of the sand grains and poor adhesion of the sand grains to the cast article; and PA1 5. Good collapsibility after casting to facilitate shakeout. There are, of course, subsidiary properties which are related to these properties, including compressive strength, permeability, compactibility, mold hardness, green shear, deformation, peel, and the like. In general, a green molding sand typically has properties within the following ranges: PA1 1. Green Tensile Strength -- Ambient and hot sand. Determined according to "AFS Foundry Sand Handbook", Sec. 8, page 6, 1963 edition. Reported in psi as the average of three tests. PA1 2. Green Compressive Strength -- Ambient sand only. Determined according to "AFS Foundry Sand Handbook", Sec. 8, page 2, 1963 edition. Reported in psi as the average of three tests. PA1 3. Dry Compressive Strength -- Ambient and hot sand. Determined according to "AFS Foundry Sand Handbook", Sec. 8, page 4, 1963 edition. Reported in psi as the average of three tests. PA1 4. Green Shear Strength -- Ambient sand only. Determined according to "AFS Foundry Sand Handbook", Sec. 8, page 5, 1963 edition. Reported in psi as the average of three tests. PA1 5. Green Permeability -- Ambient sand only. Determined according to "AFS Foundry Sand Handbook", Sec. 7, page 9, 1963 edition. Reported in permeability number as the average of three tests. PA1 6. Green Mold Hardness -- Ambient sand only. Determined according to "AFS Foundry Sand Handbook", Sec. 9, page 1, 1963 edition. Reported in mold hardness number as the average of three tests. PA1 7. Green Mold Deformation -- Ambient sand only. Determined according to "AFS Foundry Sand Handbook", Sec. 16, page 1, 1963 edition. Reported in inches per inch as the average of three tests. PA1 8. Toughness -- The product of green compressive strength and green deformation .times.10.sup.-3. PA1 9. Compactibility -- Ambient and hot sand. Determined according to "AFS Foundry Sand Handbook", Sec. 9, page 4, (Rev.-73). Reported in percent. PA1 10. Moisture -- Ambient and hot sand. Determined according to the calcium carbide method, "AFS Foundry Sand Handbook", Sec. 6, page 5, 1963 edition. PA1 11. Stick -- Hot sand only. Sand at 150.degree. F. (65.6.degree. C.) is riddled through a #6 sieve into a bronze clay wash base having a cylindrical cavity 35/8 inches in diameter and 11/8 inches deep at ambient temperature (about 70.degree. F. (21.1.degree. C.). Excess sand is struck off, the sand is allowed to stand for 3 minutes and then the mold is inverted and rapped 4 times to allow the sand to drop out. The weight of the sand, in grams, adhering to the surface of the cavity is determined by weight in grams.
______________________________________ Green Compression Strength 4 - 40 psi Green Shear Strength 0.5 - 10 psi Deformation 0.005 - 0.04 in/in Permeability 6.5 - 400 Dry Compression Strength 50 - 200.sup.+ psi Compactibility 35 - 65% ______________________________________
If the deformation or compactibility is too low, the green molding sand is too brittle and cannot withstand handling and pattern removal, while if the deformation is too high, dimensional accuracy cannot be maintained, and the mold, especially one of large mass, e.g., 100 pounds or more, may deform under its own weight. If both green strength and deformation are too high, the sand cannot be readily formed and compacted with existing technology. If permeability is less than 6.5, the vapors generated during casting cannot dissipate rapidly enough, and the mold can rupture from gas pressure and molten metal can be ejected out of the sprues. If, on the other hand, the permeability is too high, the molten metal will not be retained in the mold cavity, but will penetrate the voids of the sand. Finally, if the dry strength is too low the sand cannot withstand the erosive effect of the flowing molten metal during casting, while if the dry strength is too high the casting may crack upon solidification.
In general, foundry green molding sands consisting solely of sand, clay and water do not possess an optimum balance of properties. For this reason, a variety of additives have been employed in an effort to improve the properties of green molding sands. Typically these additives are organic materials which are used as facing agents, expansion control agents and the like. In most cases these organic additives are useful in improving only one property of the green sand and thus two or more additives may be required. In addition, an additive employed to improve one property frequently has an adverse effect on another property of the green sand mold. For example, sea coal or bituminous coal has been used as a facing agent, and while it does prevent burn-on, it has been found that increased amounts of clay and water are necessary to restore desirable physical properties possessed by the unmodified green sand.
Green molding sands may be referred to as "soft sands", because they remain plastic and re-formable throughout the mold forming procedure and, in part, during the casting operation. Such molding sands are quite distinct from other molding sands, which may be referred to as "hard sands". These "hard sands", although plastic at the beginning of the mold forming procedure, are hardened and become rigid prior to the casting operation. Hard sands are employed, for example, in investment molding processes, and in forming cores and molds made of resin-bonded sands, or sands formed of sodium silicate or phosphates, or baked drying oil sands. Such hardened sands have compression strengths of the order of 80 to 300 psi or higher. In contrast, green molding sands have compression strengths of the order of about 4 to about 40 psi, and preferably about 12 to about 30 psi.
Green molding sands also may be distinguished from "hard sands" because they are readily recycled, it being necessary only to replace temper water and, if desired, organic or other additives lost during the casting process. In contrast, hard sands can be reclaimed only by removal of all materials except for the refractory grains, and complete replacement of the bonding material. As a consequence, hard sands commonly are discarded after one use.
Because of their quite different composition and mode of use, the problems encountered in green sand casting procedures differ greatly from those of hard sand casting. One such problem is that of control of the amount of temper water to achieve adequate bond strength during both the forming and the casting steps. Slight changes in the amount of water in a green molding sand greatly affect the mechanical properties of the sand. In particular, the dry strength and the hot strength of a green molding sand depend upon the moisture of the sand at compaction; the lower the moisture content, the lower the hot and dry strengths of the sand. For example, a given percentage change in the amount of water has over five times the effect on sand strength as a similar percentage change in the amount of clay or other commonly employed green sand additive.
The problem of controlling moisture levels is compounded by a condition known as "hot green molding sand". Obviously, the sand is heated during the casting process, and unless sufficient time is allowed to elapse to allow the sand to cool to ambient temperature before re-use, the temperature of the sand increases. When the temperature of the sand reaches a temperature of from about 100 to about 160.degree. F., its physical and working properties are materially altered, making mold formation more difficult and causing casting defects. Thus, in mold formation, hot green sand sticks to the pattern and is not readily withdrawn from deep pockets. Further, sand chutes and hoppers tend to clog, and non-homogeneous mold structures are obtained as a result of variations in moisture content of the green sand. Casting defects include dirt or sand inclusions on the casting surface, blows and pinholes, erosion defects, and a general deterioration of the surface of the casting.
Without limiting the invention to any particular theory, it is believed that the primary cause of the problems encountered with hot green sand is the rapid evaporation of water from the hot sand, particularly from exposed sand surfaces both in sand transport and from formed molds, and the inability of operating personnel to control moisture content. Changes in the clay-water structure at elevated temperatures may lead to an open or gelled structural condition, which also contributes to the ease of water loss.
This rapid loss of water from hot green sand results in moisture condensing on cooler surfaces, such as the surfaces of hoppers, chutes and patterns. When these surfaces become wet, the grains in the surface layer of sand adhere more strongly to these surfaces than to other sand grains. This adhesion causes sticking in hoppers and chutes and the inability of the sand to be drawn from deep pockets of patterns. Adhesion of sand to the pattern surface leads to a roughened mold surface having exposed sand grains which are precariously attached to other sand grains. Since the surface layer of the sand loses water more rapidly, the dry strength of the bond of these surface grains to other sand grains is weaker than bonds between internal sand grains. Consequently, these exposed sand grains can be loosened by even slight jarring, and when the loosened sand grains fall and collect in the bottom of a mold cavity, they form dirt inclusions in castings made in such a mold.
In an effort to compensate for the rapid evaporation of moisture, the sand has to be prepared at higher than normal moisture levels. When this is done so that the surface sand delivered at a molding station has adequate moisture for mold forming, it has been found that the protected sand in the interior of the sand mass has an excessive moisture content, resulting in the blows and pinhole defects. This is because the excess moisture leads to the formation of excess gas when the heat from the metal enters the molding sand.
Because of the great differences between the moisture content of the surface and the interior sand due to evaporation as the sand is conveyed on belts to the molding station, a non-homogeneous sand mass results when the two are commingled in the flask of the mold. This non-homogeneous moisture condition results in a mold of non-homogeneous physical properties that has greater susceptibility to failure due to casting stress.